From, for example,
Adamson, Hingorani: "High Voltage Direct Current Power Transmission", London 1960, pp 167-171, PA0 Adamson et al: "High Voltage Direct Current Convertors & Systems", London 1965, pp 147-162, PA0 Uhlmann: "Power Transmission by Direct Current", Berlin-Heidelberg-New York 1975, hereinafter referred to as "Uhlmann", pp 361-379, and PA0 Ake Ekstrom: "High Power Electronics, HVDC and SVC, Stockholm 1990, pp 1-21 and 6-1-6-33,
it is previously well known
to use converters, connected to an ac power network, for power transmission by means of high-voltage direct current (HVDC), PA1 that a converter of the type which is generally used in such contexts generates current harmonics in that ac power network to which it is connected, PA1 that these current harmonics tend to generate interference, for example in lines for telecommunication or other signalling lines, and PA1 to connect filter equipment to the ac network to reduce the amplitude of the current harmonics and hence their disturbing influence. PA1 m=n.multidot.p.+-.1 PA1 n=1, 2, 3, . . . PA1 p is the pulse number of the converter PA1 f.sub.0 is the fundamental frequency of the ac power network (usually 50 or 60 Hz).
A converter of the kind referred to generates on its ac side current harmonics with the ordinal numbers m and the frequencies EQU f.sub.n =m.multidot.f.sub.0
where
Usually there are used in these applications converters with the pulse number 12, and such a converter generates harmonics with the ordinal numbers 11, 13, 23, 25, 35, 37, etc. The amplitude of the harmonics is, in a known manner, highest for the harmonics with the lowest ordinal numbers and decreases rapidly with increasing ordinal number.
Filter equipment previously known for damping of the above-mentioned harmonics consists of one or more three-phase filters connected to the ac network. Such a filter consists of three single-phase circuits, which are each arranged for connection between ground and a separate phase of the ac network (connection of the phase circuits between the phases of the network has also been proposed). A three-phase filter of this hitherto used type has, from an electrical and physical point of view, constituted one single unit. The filter with its three phase circuits has thus been connected to and disconnected from the network as one single unit. Likewise, the three phase circuits of the filter have been erected within a common enclosure.
For the harmonics with the lowest ordinal numbers, tuned filters are usually arranged, wherein each filter is tuned to a certain harmonic and has resonance, that is, an impedance minimum, at the frequency of this harmonic. Also double-tuned filters are used, that is, filters with impedance minima at two frequencies, and then usually at the frequencies of two adjacent harmonics, for example those with the ordinal numbers 11 and 13. Further, at harmonics with higher ordinal numbers, single-tuned filters are sometimes used for damping two adjacent harmonics, for example the harmonics with the ordinal numbers 23 and 25, the resonance frequency of the filter then being placed between the frequencies of the harmonics, in the latter example, for example, at the frequency 24.multidot.f.sub.0, and the bandwitdh of the filter being made so large that sufficient damping is obtained of the two harmonics under discussion. Such a filter is often designed with high-pass characteristic for filtering of the harmonics with the higher ordinal numbers.
The resonance frequency of a tuned filter of the kind described in the above-mentioned publications will exhibit variations, which are not negligible and which are primarily caused by the changes in capacity exhibited by the capacitances of the filter capacitors at the temperature variations to which filter equipment is subjected. The variations of the frequency of the ac network, which always occur, have the same influence. To obtain sufficient harmonic damping under all operating conditions, the filters must therefore be designed with a larger bandwitdh, that is, a lower quality factor, than what would otherwise have been necessary. To obtain a sufficiently low impedance of the filters despite this fact, it has been necessary to give them large dimensions.
The fact that the filters--which at the fundamental frequency of the network are capacitive--are to a certain extent dimensioned to contribute to the need of reactive power by the converter and possibly the network has also contributed to the large dimensions.
For the above reasons, in HVDC installations known so far, the pieces of filter equipment on the ac side have been given very large dimensions and they account for a considerable part of the total costs of an HVDC installation.
In certain cases, the filters have such large dimensions that they cannot be connected into the network or be disconnected from the network without too large voltage jumps occurring because of the connection and disconnection, respectively, of the reactive-power generating capacitors of the filters. The filters have then been divided into two or more three-phase sub-filters, which may be connected and disconnected, respectively, individually.
The strong requirement for sufficient damping of the current harmonics under all operating conditions has resulted in the necessity of rapidly disconnecting a filter (or possibly the whole filter equipment), when a fault occurs therein, and in the necessity of connecting a spare filter (or complete stand-by filter equipment). As in the case of the ordinary filter equipment, the costs and the space requirement for these spare filters are high. If the filters in the ordinary filter equipment are not divided into sub-filters, each spare filter will have the same dimensions as the ordinary filter, and the increase in cost caused by the requirement for spare filters becomes 100% of the cost of the ordinary filter. If, in the manner described above, a certain filter is divided into several mutually identical sub-filters, however, it is sufficient to arrange one single such sub-filter as a stand-by, whereby the increase in cost will be lower. Under all circumstances, however, the costs of spare filters have made up a considerable part of the total cost of a HVDC installation.
From Uhlmann, p 373, it is known that tuned filters for HVDC installations may be designed with a variable tuning by making the inductance of an inductor included in a filter mechanically controllable. This may be made automatically in that control equipment suitably senses if the resonance frequency of the filter corresponds to the frequency of the harmonic in question and varies the inductance such that correct tuning always prevails, independently of variations of the reactance values of the filter components and independently of variations of the line frequency. Alternatively, in such a filter the inductance may be controlled electromagnetically in the manner described in the international patent application PCT/SE/00946 with publication number WO 94/11891. A filter with automatic tuning may be designed with a higher factor of merit, that is, with a lower impedance, than a non-controllable filter. It will therefor have smaller dimensions and generates lower reactive power than a non-controllable filter. For this reason, it is normally not required that such a filter be divided into individually switchable three-phase sub-filters. This, however, means that a spare filter becomes as large as the ordinary filter, that is, entails an increase in cost of 100.degree..